ATP-sensitive K.sup.+ channels (K.sub.ATP) comprise a distinct family of potassium channels based on their biophysical, functional, and pharmacological characteristics. Five classes of K.sub.ATP channels are recognized based in part on differences in single channel conductance, ATP-sensitivity, pharmacology, and ion selectivity. Major properties exhibited by K.sub.ATP channels, best exemplified by the extensively characterized Type 1 channels from pancreatic beta-cells, include: reversible inhibition by intracellular ATP; rapid loss of channel activity in membrane patches following excision (channel rundown); MgATP-dependent maintenance of channel activity or reactivation following rundown in excised patches; inward rectification; and limited voltage-dependence in contrast to voltage-gated ion channels (Ashcroft et al., Cellular Signalling 2:197 (1990); Ashcroft, F. M., Ann. Rev. Neurosci. 11:97 (1988)).
Since the initial description of K.sub.ATP channels in 1983 by Noma in cardiac muscle (Noma, A., Nature 305:147 (1983)), there has been tremendous interest in the role of these metabolically-regulated channels in diverse physiological and pathophysiological processes involving a wide variety of both excitable (pancreatic .beta.-cells, central neurons, cardiac, skeletal and smooth muscle cells) and non-excitable cells (renal tubular and respiratory epithelial cells) (Ashcroft et al., Cellular Signalling 2:197 (1990); Ashcroft, F. M., Ann. Rev. Neurosci. 11:97 (1988); Lang et al., Physiol. Rev. 72:1 (1992); Kunelmann et al., Pflugers Arch. 414:297 (1989)). In part, this attention has resulted from the demonstration that sulfonylureas (e.g., tolbutamide, glibenclamide) specifically inhibit and that potassium channel openers (PCOs, e.g., diazolxide, cromakalim, pinacidil, nicroandil) activate these channels in a tissue-specific manner (de Weille et al., Pfluger Arch. 414:S80 (1989); Sanguinetti, M. C., Hypertension 19:228 (1992)). Inhibition of K.sub.ATP channels by glucose metabolism or by sulfonylureas results in cellular depolarization and the release of insulin from pancreatic .beta.-cells (Ashcroft et al., Biochem. Soc. Trans. 18:109 (1990)) and .gamma.-aminobutyric acid (GABA) from substantia nigra cells, the latter being involved in seizure control (Amoroso et al., Science 247:852 (1990)). These channels also appear to be involved in ischemia-induced alterations in cardiac myocyte electrical activity and in the regulation of smooth muscle tone. Channel activation by hypoxia, metabolic insult, or PCOs is thought to result in action potential shortening together with both antiarrhythmic and proarrhythmic effects in cardiac muscle (Nichols et al., Am. J. Physiol. 261:H1675 (1991)) and in vascular smooth muscle relaxation (Amoroso et al., Science 247:852 (1990)).
The successful molecular characterization of voltage-gated ion channels (K.sup.+, Na.sup.+ and Ca.sup.2+) (Perney et al., Curr. Opin. Cell Biol. 3:663 (1991); Stuihmer, W., Annu. Rev. Biophys. Chem. 20:65 (1991); Miller, R. J., J. Biol. Chem. 267:1403 (1992)), cyclic nucleotide-activated channels (Kaupp et al., Nature 342:762 (1989); Dhallan et al., Nature 347:184 (1990)) and more recently of a Ca.sup.2+ -activated K.sup.+ channel component (Atkinson et al., Science 253:551 (1991)) has greatly advanced our understanding of ion channel structure-function relationships and regulation and has revealed both common and distinctive features of each ion channel family. Voltage-gated Na.sup.+ and Ca.sup.2+ channel proteins contain four internal homologous domains with each domain consisting of six transmembrane segments and a pore-forming H5 region, while K.sup.+ channels are a tetrameric complex of polypeptides, each containing only one of these domains. To date, all K.sup.+ channels that have been cloned belong to either the superfamily of voltage-gated and second messenger-gated channels (Jan et al., Cell 69:715 (1992)) or to a class of channels composed of proteins with only a single membrane-spanning segment (Takumi et al., Science 242:1042 (1988)). The isolation of a K.sub.ATP channel protein or a cDNA clone, however, has remained elusive. Screening of cDNA libraries by Shaker sequence-derived oligonucleotide probes has resulted in the discovery of new members of the Shaker K.sup.+ channel family, but not in the identification of a K.sub.ATP channel. Moreover, approaches based on the affinity labelling of proteins from brain and a .beta.-cell line using sulfonylurea analog have not yielded functional channel proteins (Bernardi et al., Proc. Natl. Acad. Sci. USA 85:981 6 (1988); Aguilar-Bryan et al., J. Biol. Chem. 265:8218 (1990)). Thus given the unavailability of structural information, it has not been possible to directly address issues regarding K.sub.ATP channel gating and regulation by ATP, phosphorylation, and G protein interactions, the types and number of channel regulatory sites, the nature of the K.sub.ATP channel ion-conducting pore, and the mechanisms of action of pharmaceutical agents.